Production of paper of cellulose and polyamide fibers



United States Patent 3,354,032 PRODUCTIGN OF PAPER 0F CELLULOSE AND PGLYAMEDE FIBERS Erwin Summer, Obernburg (Main), and Klaus Gerlach,

Obernau, Germany, assignors to Vereinigte Gianzstoif- Fabriken AG., Wuppertal-Eibenfeld, Germany No Drawing. Filed Apr. 30, 1964, Ser. No. 364,012 iaims priority, appiieationofiermany, May 2, 1963, 24, 3 8 Claims. (Cl. 162146) ABSTRACT OF THE DISCLOSURE Production of a paper sheet by water-laying a mixture of cellulose and polyamide fibers from an aqueous dispersion on a paper-making machine Wherein the polyamide fibers are treated under an acid pH, prior to combination with the cellulose fibers in the dispersion, with an aqueous solution of the sodium salt of a styrene-maleic acid copolymer.

This invention relates to a sheet-like or paper-like product and the method by which it is produced. More particularly, this invention is concerned with the production of a sheet structure or paper product composed of at least about 50% by weight and preferably a major proportion of cellulose fibers in admixture with polyamide fibers by the conventional method of water-laying such materials from an aqueous fibrous slurry onto the screen or other foraminous supporting surface of a paper-making machine, and thereafter drying the water-laid web or nonwoven felt to form a flexible sheet structure. The improvement of the present invention resides in a special pretreatment of the polyamide fibers prior to their combination with the cellulose fibers during the paper-making process.

Cellulose is most often used for the production of paper or paper-like sheet structures according to the well-known paper-making process in which staple lengths of the cellulose fibers are first dispersed in water and then transferred or flowed onto a moving screen or sieve for the removal of at least part of the water and formation of the fibrous web or fleece. Thereafter, the water-laid web is dried and pressed or otherwise treated to produce the final paper product. The steps of making paper can be carried out commercially on a Fourdrinier machine or on a suitable cylinder machine for continuous production of the paper. Paper products composed entirely of cellulose fibers have been produced by this process over a long period of time, but such cellulosic papers have a relatively 10W tearing strength, a low burst pressure and unsatisfactory crease resistance. Accordingly, many attempts have been made to improve these properties in the paper product without making substantial changes in the papermakiug process and without unduly increasing the cost of paper production.

For example, it is known that certain improvements can be achieved by the addition of relatively short staple lengths of polyamide fibers to the cellulose pulp or fibrous dispersion and then water laying this mixture of cellulose and polyamide fibers. However, in order to achieve satisfactory results, a binder must be simultaneously added to the fibrous dispersion or else the water laid fibrous web must be treated with a binding resin during a subsequent stage of the process. In general, the binding agent must constitute at least by weight of the fibrous composition. Gelatin is a common binding agent which is added to the fiber dispersion so as to adhere to the fibers during water-laying on the paper-making machine. Excess water is squeezed or pressed out of the water-laid fibrous web which is then dried to remove the remaining water. This process is disadvantageous in that a very large amount of gelatin must be used, a major portion of the gelatin being led otf with the water during formation of the web on the screen of the paper-making machine. In order to avoid substantial losses of this unused gelatin, it is necessary to recycle the water from the screen in a closed circulation. This recovery and recycle problem also arises and is quite troublesome when using other binding agents which must be added to the fibrous slurry.

Another technique of adding a binding agent consists in soaking or impregnating the water-laid web of cellulose and polyamide fibers with an aqueous dispersion of a synthetic resin, and then drying and hot-pressing the web to form the final paper product. Cellulose-polyamide sheets produced in this manner generally require a large amount of the binding agent, and this high load of the binder causes the product to lose its paper-like texture. Moreover, the porosity of such products is very slight.

One process has been developed which avoids the aforementioned disadvantages and yields a paper product with good properties. In this process, the paper produced from the mixture of cellulose and polyamide fibers is impregnated with an alcoholic solution of a polyamide resin, and the impregnated paper is then dried and pressed at elevated temperatures. The use of easily combustible solvents is a disagreeable feature of this process, and furthermore, it is necessary to draw Off and condense the solvent vapors during the drying procedure. Therefore, this aftertreatment of the paper product has only a limited commercial utility.

One object of the present invention is to provide a cellulose-polyamide fibrous paper sheet of higher strength and improved crease resistance. It is also an object of the invention to achieve this improvement in properties without substantial loss of the paper-like texture and porosity of the finished product. Still another object of the invention is to provide an improved cellulose-polyamide fibrous paper product by using conventional paper-making apparatus and procedures which do not require special steps for recovering and reusing binding agent or their solvents. Yet another object of the invention is to provide the paper manufacturer with a method of producing the improved product whereby he can obtain a relatively inexpensive pretreated polyamide fiber and can then easily produce the paper product on existing equipment. These and other objects and advantages of the invention will become more apparent upon consideration of the following detailed description.

It has now been found, in accordance with the present invention, that an improved cellulose-polyamide fibrous paper or sheet-like structure can be obtained in the con-- ventional process of water-laying the ceilulose and polyamide fibers from an aqueous dispersion as a fibrous web on a paper-making machine if the polyamide fibers are pretreated under an acid pH, prior to combination with the cellulose fibers in the aqueous dispersion, with an aqueous solution of the sodium salt of a styrene-maleic acid copolymer. In general, the pie-treatment with the copolymer solution should be carried out at an acid pH of about 1.8 to 2.5, and preferably about 2.0 to 2.2. The pretreated or precoated polyamide fibers are mixed with the cellulose fibers in the preparation of the aqueous dispersion or fibrous slurry which is then water-laid on a formaminous supporting surface according to the conventional paper-making procedure. The aqueous dispersion should contain at least about 50% up to about by weight of the cellulose fibers and from about 5 to 50%, preferably 10 to 25% by weight of the pretreated polyamide fibers.

The styrene-maleic acid copolymer employed as a precoating agent for the polyamide fibers can be prepared according to conventional methods. For example, the copolymer can be prepared by free radical polymerization of styrene and maleic acid anhydride as co-monomers in a molar ratio of about 0.5 :1 to 1:0.5, preferably in an approximately equimolar ratio of styrene to maleic acid anhydride. Subsequent treatment with sodium hydroxide renders the copolymer alkali-soluble. According to one specific method, the styrene and maleic acid anhydride are copolymerized in acetone in the presence of a free radical catalyst such as benzoyl peroxide, and the resultingproduct is reacted with normal NaOH at about 75 C. to give the water-soluble sodium salt of the copolymer. The copolymer preferably has a molecular weight on the order of at least 10,000 up to about 250,000.

The polyamide fibers are those obtained from the usual fiber-forming high molecular weight polyamides such as polyhexamethylene adipamide or caprolactam, the fiberforming polymer being spun and then stretched as a monofilarnent and subsequently cut into a staple length suitable for felting or forming a water-laid web, for example staple lengths ofabout 2 to mm. The methods of forming these polyamide filaments and their general use in the paper-making process are so well-known as to require no further elaboration here.

The precoating or pretreating of the polyamide filaments or fibers'can be easily accomplished in a single step by passing the polyamide filaments as a yarn or bundle through an aqueous solution of the styrene-maleic acid copolymer salt. The treatment solution or bath is prepared by first dissolving the alkali-soluble copolymer salt in water, and it is essential that this solution be acidified in this one step procedure, preferably to a pH value just short of that at which the copolymer will recipitateas the free acid, this precipitation occurring at a pH value of about 1.8. It is also possible to apply the acidified copolymer salt solution to the polyamide fibers either before or after cutting the filaments into shorter or staple lengths.

Instead of treating the polyamide fibers in a one-bath process with an aqueous acidified solution of the copolymer salt, the pretreatment can also be carried out as a two-bath process. This two-bath process is accomplished, for example, by first drawing the polyamide filaments or yarn through a dilute acid bath, then squeezing or otherwise removing excess bath liquid from the filaments and thereafter conducting the filaments through a non-acidified aqueous solution of the copolymer salt. In order to obtain uniform results, the pH value of the non-acidified copolymer salt solutions should be maintained at a constant level by the continuous addition of an alkaline compound such as sodium hydroxide. This two-bath technique substantially avoids any danger of copolymer recipitation which might be caused by an overadjustment of the pH in thesingle bath treatment. In other words, the two-bath method avoids the careful adjustment of an acid pH value of the copolymer salt solution, but the preliminary dilute acid treatment does produce an acid condition at the surface of the filaments even though they are subsequently drawn through a neutral to alkaline solution of the copolymer salt.

Any acid may be used in the process of the invention, i.e. all inorganic and organic acids are suitable, whether one employs the single bath method or the double bath method. In the single bath method, the pH value of the copolymer salt solution is simply adjusted to a value of :about 1.8 to 2.5, preferably 2.0 to 2.2, and this same condition is approximated when coating the filaments with dilute acid in the two-bath method. The amount of acid to be supplied in any case can be readily determined by simple preliminary tests.

After the polyamide filaments have been pretreated with the copolymer salt solution, excess bath liquid can be removed and the filaments can then be dried or can be immediately used as moist fibers in the paper-making process In either ease, the precoated polyamide fibers can be readily dispersed and mixed together with the cellulose fibers in the head box of the paper-making machine. Thereafter, the slurry or pulp of precoated polyamide fibers and cellulose fibers is water-laid into a fibrous web and the web dried to form a paper sheet according to the conventional paper-making technique. It is not necessary to add additional binding agents to the fibrous web, and the drying of the web can take place with or without pressure at temperatures of 100 C. or at lower temperatures.

In both the single bath and the double bath method of the invention, the aqueous solution of the styrenemaleic acid copolymer salt should have a copolymer concentration of about 0.1 to 1.0% by weight, preferably 0.3 to 0.7% by Weight. The bath temperatures, whether single or double, can be maintained at a temperature of about 40 to 85 C., higher temperatures permitting shorter treatment periods. The pretreated polyamide fibers should normally have an adherent coating of the styrene-maleic acid copolymer of about 0.5 to 3.0% by Weight, with the same content of the copolymer appearing in the final paper product. It will be understood that these amounts of the copolymer can be varied depending upon the particular quality and strength properties desired in the final product. I

The following examples will further illustrate the invention and provide a comparison with untreated polyamide fibers. Parts and percentages are by weight unless otherwise indicated.

Example 1 A polyamide continuous filament yarn having a total denier of 86,000 and an individual denier of 1.4 was obtained by spinning and stretching from slit orifices having individual dimensions of 60 x 800g. This yarn was immersed in a 0.5% aqueous solution of the sodium salt of a copolymer of styrene and maleic acid anhydride combined in a molar ratio of 1:1. The copolymer had a molecular weight of about 100,000. The copolymer solution was previously acidified by addition of sulfuric acid in order to obtain a pH value of 2.2, and the polyamide yarn was immersed in this solution for a period of one minute at C. Excess oath liquid was then squeezed from the yarn between two rollers, and the filaments were cut to a staple length of 6 mm. One part of the fibers pretreated in this manner was then dispersed in water together with nine parts of sulfite cellulose which had been preground to 30 Schopper-Riegler. The resulting fibrous slurry contained 10,000 parts of water to 10 parts of fibers and was constantly agitated while being water-laid on a laboratory sheet former in order to provide individual sheets of 70 to 80 g./m. The water-laid fibrous web or sheet was dried at 96 C. in a conventional sheet dryer. Additional sheets were produced in an identical manner so as to contain a pretreated polyamide fiber content of 20% by weight. The properties of the resulting paper products are shown in the table below.

Example 2 A polyamide filament yarn as described in Example 1 was dipped for one minute in 0.05 N sulfuric acid .at 80 C., squeezed between rollers to remove excess acid and finally treated with an aqueous and non-acidified solution of the styrene-maleic acid copolymer in the same manner as in Example 1, ie for one minute at 80 C. After cutting the pretreated polyamide filaments to the staple length of 6 mm., a fibrous slurry was again formed with cellulose fibers and water laid and dried as in Example 1 to provide paper sheets containing 20% by weight of the polyamide fibers. The properties of the paper products obtained by this two-bath method .are shown in the table below.

Example 3 For purposes of comparison and control, additional paper samples were prepared by the same paper-making procedure followed in the previous examples, but using cellulose fibers alone or in combination with 10% by weight and 20% by weight of untreated polyamide fibers. The results of these control tests are also shown in the 2. A process as claimed in claim 1 wherein said pretreatment is carried out at an acid pH of about 1.8 to 2.5.

3. A process as claimed in claim 1 wherein said pretreatment is carried out at an acid pH of about 2.0 to 2.2.

table below. 5 4. A process as claimed in claim 2 wherein said co- TABLE Poly- Surface Tearin Bursting Tear-away Ex. amide weight, length, Stretch, pressure, Creasing strength,

fiber, gJm. in. percent kg./cml N 0. cm. g./crn. percent 1 No longer measurable with the Brecht-Imset apparatus.

In accordance with the preceding table and examples, it will be noted that the polyamide-cellulose fibrous paper product of the invention provides a substantial improvement in the product, even when using a smaller content of pretreated polyamide fibers when compared to the use of untreated polyamide fibers. The quality and appearance of the paper product is maintained within desirable standards, and the crease resistance shows a very considerable improvement.

An especially advantageous feature of the invention resides in the fact that the pretreated polyamide fibers can be stored either in the dry state or in the moist state after application of the styrene-maleic acid copolymer salt. The pretreated polyamide fibers do not stick together and can be subsequently dispersed without any difiiculty when forming the fibrous slurry in water together with cellulose fibers. The paper manufacturer can thus obtain the pretreated polyamide fibers and admix them with cellulose fibers in any conventional papermaking process, .and it is unnecessary to use any special equipment or handle any chemicals such as solvents or the like during the paper-making process. Finally, the precoating of the polyamide fibers can be accomplished in a relatively simple manner so that the improved results far outweigh any slight additional cost. The paper products of this invention have a Wide range of utility wherever paper is used under conditions requiring high strength and good crease resistance. Specific applications of polyamide-cellulose papers are well known in the art.

As will be apparent to those skilled in the art, the paper-making process itself can be widely modified and minor or equivalent modifications can also be made in the above described pretreatment of the polyamide fibers without departing from the spirit or scope of the present invention.

The invention is hereby claimed as follows:

1. In a process for the production of a paper sheet from an aqueous dispersion of cellulose and polyamide fibers by water-laying said fibers as a fibrous web on a paper-making machine, the improvement which comprises: pretreating said polyamide fibers under an acid pH, prior to combination with the cellulose fibers in said aqueous dispersion, with an aqueous solution of the sodium salt of a styrene-maleic acid copolymer.

polymer has a molar ratio of styrene to maleic acid of about 0.5:1 to 1:05.

5. A process as claimed in claim 2 wherein the pretreated polyamide fibers are dried prior to combination with said cellulose fibers.

6. A process for the production of a paper sheet having a fibrous content of about 50 to by weight of cellulose fibers and about 5 to 50% by weight of polyamide fibers, which process comprises: precoating said polyamide fibers with a styrene-maleic acid copolymer by immersing the fibrous poly-amide in an aqueous solution maintained at a pH of about 1.8 to 2.5 of the sodium salt of said copolymer; mixing the precoated polyamide fibers with the cellulose fibers into a fibrous slurry; waterlaying the mixture of polyamide-cellulose fibers into a fibrous web; and drying the fibrous web to form a paper sheet.

7. A process for the production of a paper sheet having a fibrous content of about 50 to 95 by weight of cellulose fibers and about 5 to 50% by weight of polyamide fibers, which process comprises: coating the fibrous polyamide with aqueous acid and removing excess liquid in a first stage; immersing the acidified fibrous polyamide in a non-acidified aqueous solution of the sodium salt of a styrene-maleic .acid copolymer in a second stage; mixing the resulting two-stage pretreated polyamide fibers with the cellulose fibers into a fibrous slurry; water-laying the mixture of polyamide-cellulose fibers into a fibrous web; and drying the fibrous web to form a paper sheet.

8. The paper sheet obtained by the process of claim 1.

References Cited UNITED STATES PATENTS 3,095,345 6/1963 Jackson et .al. l62146 3,103,462 9/1963 Griggs et al. 162-136 OTHER REFERENCES Paper Trade Journal, Synthetic-Fiber Papers Proved in Pilot Mill Runs, pp. 38 and 40, Vol. 140, Mar. 5, 1956.

S. LEON BASHORE, Acting Primary Examiner,

HOWARD R. CAINE, Examiner, 

1. IN A PROCESS FOR THE PRODUCTION OF A PAPER SHEET FROM AN AQUEOUS DISPERSION OF CELLULOSE AND POLYAMIDE FIBERS BY WATER-LAYING SAID FIBERS AS A FIBROUS WEB ON A PAPER-MAKING MACHINE, THE IMPROVEMENT WHICH COMPRISES: PRETREATING SAID POLYAMIDE FIBERS UNDER AN ACID PH, PRIOR TO COMBINATION WITH THE CELLULOSE FIBERS IN SAID AQUEOUS DISPERSION, WITH AN AQUEOUS SOLUTION OF THE SODIUM SALT OF A STYRENE-MALEIC ACID COPOLYMER. 